One challenge facing the underground mining industry is the instability of rock mass comprising the roof and walls of mines. Rock mass may shift and/or loosen over time, increasing the likelihood of rock falls. To lessen the likelihood and impact of rock falls, rock bolts may be driven into bore holes in the rock mass. Rock bolts typically comprises rigid substances, such as metal or hard plastic and may vary in length—lengths of eighteen inches to over twenty feet are common. Rock bolts are typically formed as cylinders and may have a solid or hollow core.
In addition to providing stability to the rock mass, rock bolts facilitate the detection of potentially hazardous stresses and strains in the rock mass. Strain gauges affixed to the rock bolts provide a measure of the strains and hence the stresses which the rock bolt is subjected to. However, attempts to fit rock bolts with strain gauges have been problematic. Affixing strain gauges to the outside surface of rock bolts is largely impractical, due to the tendency of strain gauges to be damaged or dislocated from the rock bolt when the rock bolt is inserted into the bore hole. Fitting strain gauges within closed hollow-core rock bolts also presents a challenge, due to the inaccessibility of the interior core of such bolts.
Strain gauges must typically be energized via conductors in order to produce signals under strain. Energizing strain gauges affixed to rock bolts that are inserted into bore holes, and retrieving signals from these gauges, has proven problematic. When the gauge's conductors are exposed outside of the rock bolt, they may be damaged and degraded by the harsh conditions present in mines.